Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.
One particular LPP technique involves irradiating a target material droplet with one or more pre-pulse(s) followed by a main pulse. In this regard, CO2 lasers may present certain advantages as a drive laser producing “main” pulses in an LPP process. This may be especially true for certain target materials such as molten tin droplets. For example, one advantage may include the ability to produce a relatively high conversion efficiency e.g., the ratio of output EUV in-band power to drive laser input power.
In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In more quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 40,000-100,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g. 40-100 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position (i.e. with very small “jitter”) over relatively long periods of time.
For a typical LPP setup, target material droplets are generated and then travel within a vacuum chamber to an irradiation site where they are irradiated, e.g. by a focused laser beam. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber (e.g. debris) that can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. These debris can include high-energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of is source material. For this reason, it is often desirable to use so-called “mass limited” droplets of source material to reduce or eliminate the formation of debris. The use of “mass limited” droplets also may result in a reduction in source material consumption. Techniques to achieve a mass-limited droplet may involve diluting the source material and/or using relatively small droplets. For example, the use of droplets as small as 10-50 μm is currently contemplated.
In addition to their effect on optical elements in the vacuum chamber, the plasma by-products may also adversely affect the droplet(s) approaching the irradiation site (i.e. subsequent droplets in the droplet stream). In some cases, interactions between droplets and the plasma by-products may result in a lower EUV output for these droplets. In this regard, U.S. Pat. No. 6,855,943 (hereinafter the '943 patent) which issued to Shields on Feb. 15, 2005 and is entitled “DROPLET TARGET DELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE” discloses a technique in which only some of the droplets in a droplet stream, e.g., every third droplet, is irradiated to produce a pulsed EUV light output. As disclosed in the '943 patent, the nonparticipating droplets (so-called buffer droplets) advantageously shield the next participating droplet from the effects of the plasma generated at the irradiation site. However, the use of buffer droplets may increase source material consumption and/or vacuum chamber contamination and/or may require droplet generation at a frequency much higher (e.g. by a factor of two or more) than required without the use of buffer droplets. On the other hand, if the spacing between droplets can be increased, the use of buffer droplets may be reduced or eliminated. Thus, droplet size, spacing and timing consistency (i.e. jitter) tend to be on the top of the list of factors to be considered when designing a droplet generator for an LPP EUV light source.
One technique for generating droplets involves melting a target material, e.g. tin, and then forcing it under high pressure through a relative small diameter orifice, e.g. 5-30 μm. Under most conditions, naturally occurring instabilities, e.g. noise, in the stream exiting the orifice may cause the stream to break up into droplets. In order to synchronize the droplets with optical pulses of the LPP drive laser, a repetitive disturbance with an amplitude exceeding that of the random noise may be applied to the continuous stream. By applying a disturbance at the same frequency (or its higher harmonics) as the repetition rate of the pulsed laser, the droplets can be synchronized with the laser pulses. In the past, the disturbance has typically been applied to the stream by driving an electro-actuatable element (such as a piezoelectric material) with a waveform of a single frequency such as a sinusoidal waveform, triangular waveform, square waveform or their equivalent.
As used herein, the term “electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes but is not limited to piezoelectric materials, electrostrictive materials and magnetostrictive materials.
In general, for the application of single frequency, non-modulated waveform disturbances, the spacing between droplets increases as the disturbance frequency decreases (i.e. holding other factors such as pressure and orifice diameter constant). However, as disclosed in “Drop Formation From A Vibrating Orifice Generator Driven By Modulated Electrical Signals” (G. Brenn and U. Lackermeier, Phys. Fluids 9, 3658 (1997) the contents of which are incorporated by reference herein), for disturbance frequencies below about 0.3ν/(πd), where ν is the stream velocity and d is the diameter of the continuous liquid stream, more than one droplet may be generated for each disturbance period. Thus, for 10 μm liquid jet at a stream velocity of about 50 m/s, the calculated frequency minimum below which more than one drop per period may be produced is about 480 kHz (note: it/is currently envisioned that a droplet repetition rate of 40-100 kHz and velocities of about 30-50 m/s may be desirable for LPP EUV processes). The net result is that is for the application of single frequency, non-modulated waveform disturbances, the spacing between droplets is fundamentally limited and cannot exceed approximately 3.33 πd. As indicated above, it may be desirable to supply a sufficient distance between adjacent droplets in the droplet stream to reduce/eliminate the effect of the debris from the plasma on approaching droplet(s). Moreover, because the limitation on spacing is proportional to stream diameter, and as a consequence droplet size, this limitation can be particularly severe in applications such as LPP EUV light sources where relatively small, mass-limited, droplets are desirable (see discussion above).
With the above in mind, Applicants disclose a laser produced plasma, EUV light source having a droplet stream produced using a modulated disturbance waveform, and corresponding methods of use.